No abstract

We present results from field experiments linking hydrology, geochemistry, and microbiology during infiltration at a field site that is used for managed aquifer recharge (MAR). These experiments measured how a horizontal permeable reactive barrier (PRB) made of woodchips impacted subsurface nitrate removal and microbial ecology. Concentrations of dissolved organic carbon consistently increased in infiltrating water below the PRB, but not in un-amended native soil. The average nitrate removal rate in soils below the PRB was 1.5 g/m/day NO-N, despite rapid infiltration (up to 1.9 m/d) and a short fluid residence time within the woodchips (≤6 h). In contrast, 0.09 g/m/day NO-N was removed on average in native soil. Residual nitrate in infiltrating water below the PRB was enriched in δN and δO, with low and variable isotopic enrichment factors that are consistent with denitrification during rapid infiltration. Many putative denitrifying bacteria were significantly enhanced in the soil below a PRB; Methylotenera mobilis and genera Microbacterium, Polaromonas, and Novosphingobium had log fold-changes of +4.9, +5.6, +7.2, and +11.8, respectively. These bacteria were present before infiltration and were not enhanced in native soil. It appears that the woodchip PRB contributed to favorable conditions in the underlying soil for enhanced nitrate removal, quantitatively shifting soil microbial ecology. These results suggest that using a horizontal PRB could improve water quality during rapid infiltration for MAR.

We present linked field and laboratory studies investigating controls on enhanced nitrate processing during infiltration for managed aquifer recharge. We examine how carbon-rich permeable reactive barriers (PRBs) made of woodchips or biochar, placed in the path of infiltrating water, stimulate microbial denitrification. In field studies with infiltration of 0.2–0.3 m/day and initial nitrate concentrations of [NO3-N] = 20–28 mg/L, we observed that woodchips promoted 37 ± 6.6% nitrate removal (primarily via denitrification), and biochar promoted 33 ± 12% nitrate removal (likely via denitrification and physical absorption effects). In contrast, unamended soil at the same site generated <5% denitrification. We find that the presence of a carbon-rich PRB has a modest effect on the underlying soil microbial community structure in these experiments, indicating that existing consortia have the capability to carry out denitrification given favorable conditions. In laboratory studies using intact cores from the same site, we extend the results to quantify how infiltration rate influences denitrification, with and without a carbon-rich PRB. We find that the influence of both PRB materials is diminished at higher infiltration rates (>0.7 m/day) but can still result in denitrification. These results demonstrate a quantitative relationship between infiltration rate and denitrification that depends on the presence and nature of a PRB. Combined results from these field and laboratory experiments, with complementary studies of denitrification during infiltration through other soils, suggest a framework for understanding linked hydrologic and chemical controls on microbial denitrification (and potentially other redox-sensitive processes) that could improve water quality during managed recharge.

We quantified the distribution of hillslope runoff under different climate and land-use conditions in a coastal, mixed land-use basin, the Pajaro Valley Drainage Basin (PVDB), California, USA, in order to evaluate opportunities to improve groundwater supply. We developed dry, normal, and wet climate scenarios using high-resolution historic data and compared contemporary land use to pre-development land use under the different climate scenarios. Relative to pre-development conditions, urban and agricultural development resulted in more than twice as much simulated runoff generation, greater spatial variability in runoff, and less water available for recharge; these differences were most pronounced during the dry climate scenario. Runoff results were considered in terms of potential to support distributed stormwater collection linked to managed aquifer recharge (DSC-MAR), which routes excess hillslope runoff to sites where it can infiltrate and enhance groundwater recharge. In the PVDB, 10% of the annual groundwater deficit could be addressed by recharging 4.3% of basin-wide hillslope runoff generated during the normal scenario, and 10.0% and 1.5% of runoff during the dry and wet scenarios, respectively. Runoff simulation results were combined with an independent recharge suitability mapping analysis, showing that DSC-MAR could be effective in many parts of the PVDB under a range of climate conditions. These results highlight the importance of strategically locating DSC-MAR projects at the confluence of reliable supply and favorable subsurface hydrologic properties.

Stream temperature is a critical water quality parameter that is not fully understood, particularly in urban areas. This study explores drivers contributing to stream temperature variability within an urban system, at 21 sites within the Philadelphia region, Pennsylvania, USA. A comprehensive set of temperature metrics were evaluated, including temperature sensitivity, daily maximum temperatures, time >20 C, and temperature surges during storms. Wastewater treatment plants (WWTPs) were the strongest driver of downstream temperature variability along 32 km in Wissahickon Creek. WWTP effluent temperature controlled local (1-3 km downstream) temperatures year-round, but the impacts varied seasonally: during winter, local warming of 2-7 C was consistently observed, while local cooling up to 1 C occurred during summer. Summer cooling and winter warming were detected up to 12 km downstream of a WWTP. Comparing effects from different WWTPs provided guidelines for mitigating their thermal impact; WWTPs that discharged into larger streams, had cooler effluent, or had lower discharge had less effect on stream temperatures. Comparing thermal regimes in four urban headwater streams, sites with more local riparian canopy had cooler maximum temperatures by up to 1.5 C, had lower temperature sensitivity, and spent less time at high temperatures, although mean temperatures were unaffected. Watershed-scale impervious area was associated with increased surge frequency and magnitude at headwater sites, but most storms did not result in a surge and most surges had a low magnitude. These results suggest that maintaining or restoring riparian canopy in urban settings will have a larger impact on stream temperatures than stormwater management that treats impervious area. Mitigation efforts may be most impactful at urban headwater sites, which are particularly vulnerable to stream temperature disruptions. It is vital that stream temperature impacts are considered when planning stormwater management or stream restoration projects, and the appropriate metrics need to be considered when assessing impacts.

In this study, we conducted a meta-analysis of soil microbial communities at three, pilot-scale field sites simulating shallow infiltration for managed aquifer recharge (MAR). We evaluated shifts in microbial communities after infiltration across site location, through different soils, with and without carbon-rich amendments added to test plots. Our meta-analysis aims to enable more effective MAR basin design by identifying potentially important interactions between soil physical-geochemical parameters and microbial communities across several geographically separate MAR basins. We hypothesized infiltration and carbon amendments would lead to common changes in subsurface microbial communities at multiple field sites but found distinct differences instead. Sites with coarser (mainly sandy) soil had large changes in diversity and taxa abundance, while sites with finer soils had fewer significant changes in genera, despite having the greatest increase in nitrogen cycling. Below plots amended with a carbon-rich permeable reactive barrier, we observed more nitrate removal and a decrease in genera capable of nitrification. Multivariate statistics determined that the soil texture (a proxy for numerous soil characteristics) was the main determinant of whether the microbial community composition changed because of infiltration. These results suggest that microbial communities in sandy soil with carbon-rich amendments are most impacted by infiltration. Soil composition is a critical parameter that links between microbial communities and nutrient cycling during infiltration and could influence the citing and operation of MAR to benefit water quality and supply.

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Managed aquifer recharge (MAR) can increase groundwater supply and, under some conditions, improve groundwater quality simultaneously. However, the conditions under which water quality improvements can be achieved during infiltration for MAR have not been systematically examined in a spatially explicit manner. This work aims to address that gap by synthesizing observations from laboratory tests, field experiments and operational facilities at four MAR sites within the Pajaro Valley, California, USA to develop a predictive model of nitrate removal during infiltration. We compare two machine learning approaches; random forest and boosted regression trees, to multiple linear regression. The preferred model uses boosted regression trees, with four predictor variables (initial nitrate concentration of infiltrating water, residence time in the soil, soil organic carbon content, and soil percent clay). We apply this model to simulate the spatial distribution of potential nitrate removal (NRp) across a heterogeneous and mixed‐use landscape, finding that >87% of the region has the potential to remove nitrate during infiltration. To link potential nitrate removal capacity to available water for MAR, we combine a map of NRp with independently simulated hillslope runoff, and find that potential load reduction is highest in urban areas (median = 18.8 kg‐N/yr) where large runoff volumes are collocated with soils having high nutrient cycling capacity compared to forested and agricultural areas (median = 1.6 and 3.5 kg‐N/yr, respectively). We analyse potential load reduction across dry, normal, and wet precipitation scenarios, which shows that urban areas have the potential to yield large load reductions (median = 11.3 kg‐N/yr) even under relatively dry conditions. However, much of the generation of elevated nitrate in runoff is associated with agricultural activity. These results suggest that the urban–agricultural interface represents a crucial link between hydrologic and biogeochemical cycles where water supply and quality goals may be met simultaneously. The technical approach used in this study is highly flexible and can help guide decisions in resource management and identify promising MAR sites. More broadly, this approach also elucidates heterogeneity in biogeochemical cycling across complex landscapes.